Electrode for electrolytic evolution of gas

ABSTRACT

An electrode for evolution of gas in electrolytic processes having a substrate of valve metal and a catalytic coating having two layers. A first layer having oxides of valve metal, ruthenium and iridium and a second layer having one or more metals chosen from amongst elements of the platinum group.

FIELD OF THE INVENTION

The invention relates to an electrode for evolution of gas in electrolytic processes comprising a valve metal substrate and a catalytic coating comprising two layers. A first layer comprising valve metal, ruthenium and iridium oxides and a second layer comprising one or more metals chosen from amongst the elements of the platinum group.

BACKGROUND OF THE INVENTION

The field of the invention relates to the preparation of a catalytic coating for electrodes used in brine electrolysis processes. This coating is applied to a metal substrate, typically titanium or other valve metal.

Over the years, the technology of brine electrolysis has undergone innovations towards an efficient implementation from the energy point of view and from the cost/benefit of the use of resources. In this ever more challenging context, the optimization of the anode plays a key role. In particular, numerous efforts have been made in order to reduce the over-voltage of the anode in the generation of chlorine and in order to reduce the concentration of oxygen in the gaseous chlorine generated and thus to produce gaseous chlorine with a high purity.

A further difficulty resides in the obtaining of an electrode capable of maintaining higher performance for a long period of time.

Generally speaking, the processes for electrolysis of brines, for example alkaline chloride brines such as sodium chloride, for the production of chlorine and caustic soda, are carried out with anodes made of titanium or another valve metal, activated with a superficial layer of ruthenium dioxide (RuO2) optionally mixed with tin dioxide (SnO2) and another noble metal, such as for example described in EP0153586. Accordingly, it is possible to obtain a decrease in the over-voltage of the chlorine evolution anodic reaction and thus in the overall energy consumption.

The formulation just described, together with the other formulations containing tin, has however the problem of also reducing the over-voltage of the concurrent oxygen development reaction, leading to the production of chlorine gas contaminated with an excessive quantity of oxygen.

Another partial improvement in the performance is obtained by applying to a metal substrate a formulation based on RuO2 and SnO2 combined with a reduced quantity of IrO2 such as for example described in WO2016083319. A similar formulation allows optimum values of cell potential and moderate quantities of oxygen to be obtained.

Other coatings of the prior art, such as for example the formulation described in WO2012081635 comprising two catalytic coatings, the first containing titanium and noble metal oxides and the second containing a platinum and palladium alloy, also allow optimum values of cell potential and reduced quantities of oxygen in chlorine gas to be obtained; however, they do not endow the electrode with an optimum resistance capable of maintaining higher levels of performance, with regard to catalytic activity and selectivity, for an adequate period of time.

US 2013/0186750 A1 describes an electrode suitable for chlorine evolution which has alternate layers of two distinct compositions, namely one type of layers comprising iridium, ruthenium and valve metals and another type of layer comprising oxides of iridium, ruthenium and tin.

US 2013/0334037 A1 describes an electrode for electrolysis including a conductive substrate, a first layer formed on the conductive substrate containing at least one oxide selected from ruthenium oxide, iridium oxide and titanium oxide and a second layer formed on the first layer containing an alloy of platinum and palladium.

U.S. Pat. No. 4,626,334 describes an anode comprising an electroconductive substrate provided with a (Ru—Sn)O2 solid solution coating for brine electrolysis.

JP S62243790 describes an electrode having a first coating layer comprising a mixture of platinum and iridium oxide and a second coating layer comprising a mixture of ruthenium oxide and tin oxide.

The need is thus apparent to identify a new catalytic coating for electrodes for evolution of gaseous products in electrolytic cells in brine electrolysis processes, characterized by a higher level of catalytic activity and by a high resistance capable of sustaining higher levels of performance for a long period of time under the usual operating conditions with respect to the formulations of the prior art.

SUMMARY OF THE INVENTION

Various aspects of the present invention are described in the appended claims. The present invention relates to an electrode for evolution of gaseous products in electrolytic cells, for example for evolution of chlorine in alkaline brine electrolysis cells, comprising a catalytic coating applied on a metal substrate. In the present context, the term catalytic coating indicates two different catalytic layers with different catalytic compositions in which the first catalytic layer formed on the substrate comprises at least a mixture of iridium, of ruthenium, of tin and of platinum or their oxides or respective combinations and a second catalytic layer formed on the first catalytic layer comprises platinum and tin or their oxides or respective combinations thereof. The tin of the second catalytic layer is present in a concentration decreasing from the interface with said first catalytic layer towards the upper surface of the second catalytic layer, i.e. surface opposite the interface with the first catalytic layer, and the platinum of the said first catalytic layer is present in a concentration decreasing from the interface with said second catalytic layer towards the substrate.

The present invention also relates to an electrode for evolution of gaseous products in electrolytic cells, for example for evolution of chlorine in alkaline brine electrolysis cells, comprising a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate containing a mixture of iridium, ruthenium, tin and platinum or their oxides or combinations thereof and a second catalytic layer formed on said first catalytic layer containing platinum and tin or their oxides or combinations thereof, wherein said first layer is obtained from a platinum-free first precursor solution comprising a mixture of iridium, ruthenium and tin, applied said substrate and subjected to a heat treatment, and wherein said second catalytic layer is obtained from a tin-free second catalytic solution containing platinum, applied to said first catalytic layer and subjected to a heat treatment. The terms “platinum-free” and “tin-free” in the sense of the present invention mean that the platinum concentration in the first solution is at least an order of magnitude lower that the average platinum concentration in the first layer obtained from said first solution and that the tin concentration in the second solution is at least an order of magnitude lower that the average tin concentration in second layer obtained from the second solution. Preferably, a platinum-free solution contains platinum at most as an impurity and a tin-free solution contains tin at most as an impurity.

This double-layer structure, applied to a metal substrate, typically titanium, titanium alloy or another valve metal, allows a saving in the energy consumption to be combined with an excellent purity of chlorine gas produced while maintaining optimal performance characteristics in terms of catalytic activity and of selectivity for a long period of time.

The first catalytic layer, formed on the substrate, preferably comprises ruthenium oxide, iridium oxide, tin oxide and metallic platinum or its oxides. RuO2 is widely known for its excellent catalytic activity and its stability in an alkaline medium which is improved by the presence of IrO2; the presence of SnO2 guarantees a slower consumption of the noble metals present.

The second catalytic layer, formed on the first layer, comprises tin or its oxides and one or more metals chosen from amongst the elements of the platinum group, especially platinum itself, which are known for increasing the selectivity and for reducing the energy consumption.

The inventors have observed that an electrode with a similar catalytic coating, where said second catalytic layer comprises platinum in a molar percentage referred to the metal element in the range between 48 and 96% (or, when taking the tin component not into account, from 50 and 99.999%) in the form of the metal or its oxide, can offer the advantage of subsequently reducing the over-voltage of the reaction for evolution of chlorine.

In the context of the present invention, ranges denoted by either “from” or “between” include the specified upper and lower limits, respectively.

In another embodiment, aside from platinum and tin, said second catalytic layer comprises palladium or rhodium in the form of metals or their oxides, or combinations thereof, in molar percentage referred to the metal elements in the range between 0 and 24% (or, when taking the tin component not into account, between 0 and 25%), where the elements are in the form of metals or oxides thereof. This can guarantee a high catalytic activity by virtue of the combined presence of two or more noble metals.

The second catalytic layer preferably comprises tin or its oxide in an average molar percentage referred to the metal element in a range from 4 to 12%. As the concentration of the tin component varies in a direction perpendicular to the interface between the first and second layers, the tin concentration is an average of the concentration profile through the second catalytic layer.

Therefore, in a preferred embodiment, besides unavoidable impurities, the second catalytic layer consists of platinum and tin, and optionally palladium and/or rhodium, in molar percentage referred to the metal elements in the ranges from 48 to 96% platinum, from 4 to 12% tin, from 0 to 24% palladium and from 0 to 24% rhodium.

According to a preferred embodiment of the aforementioned electrode, the first catalytic layer comprises metals or metal oxides of iridium, ruthenium, tin in molar percentages Ru=24-34%, Ir=3-13%, Sn=30-70% referred to the metal elements.

The first catalytic layer preferably comprises platinum or its oxide in an average molar percentage referred to the metal element in a range from 3 to 10%. As the concentration of the platinum component varies in a direction perpendicular to the interface between the first and second layers, the platinum concentration is an average of the concentration profile through the first catalytic layer.

It goes without saying that those skilled in the art will select the molar percentages of the individual elements in such a manner that the total sum of the molar percentages of the components is 100. Especially, if no other metals are present in the first catalytic layer, Sn or Sn oxides are preferably present in concentration of 55-70% referred to the metal element.

In another embodiment, said first catalytic layer comprises another valve metal chosen from amongst titanium, tantalum and niobium, in a quantity, expressed in molar percentage, in the range between 30 and 40% referred to the metal element; it has in fact been observed how the presence of another valve metal such as titanium allows a good catalytic activity to be combined with a substantial increase in the resistance of the electrode in processes that require current inversion.

In a preferred embodiment, besides unavoidable impurities, the first catalytic layer consists of iridium, ruthenium, tin and platinum and optionally titanium, in molar percentage referred to the metal elements in the ranges from 3 to 13% iridium, from 24 to 34% ruthenium, from 30 to 70% tin, from 3 to 10% platinum and from 30 to 40% titanium.

The inventors have observed that, surprisingly, in the catalytic coating described above, a phenomenon of diffusion between layers takes place: the tin of the first catalytic layer diffuses into the second layer, while the platinum of the second catalytic layer diffuses into the first layer. The diffusion of tin into the second catalytic layer takes place across a gradient of concentration such that the quantity of tin in the second catalytic layer is maximum at the interface between the two catalytic layers and decreases towards the external surface of the second catalytic layer.

The presence of tin diffused into the second catalytic layer can advantageously slow the consumption of the noble metals present in the second catalytic layer, enabling optimum performance characteristics in terms of catalytic activity and of selectivity to be maintained for a longer period of time, without compromising the catalytic performance.

Likewise, the diffusion of platinum from the second catalytic layer into the first catalytic layer is such that the quantity of platinum in the first catalytic layer is maximum at the interface between the two catalytic layers and decreases gradually toward the internal surface of the first catalytic layer.

The diffusion of the platinum into the first catalytic layer allows the catalytic activity to be enhanced. This furthermore allows better catalytic performance characteristics to be maintained throughout the lifetime of the electrode, also where the prolonged use of the same causes wear of the second layer over time. The elements present and the particular structure of the catalytic coating allow better performance characteristics with respect to the prior art to be guaranteed with the further advantage of increasing the operating lifetime of the electrode.

The electrode according to the invention furthermore surprisingly allows the better performance characteristics in terms of activity and of selectivity to be maintained over time.

The presence of tin has a high impact on the selectivity; however, if the tin is present in high quantities on the external surface of the catalytic coating, in combination with the platinum, it attenuates the increase in catalytic activity of the platinum itself.

The diffusion of tin from the first catalytic layer to the second produces a profile of concentration of the element between the layers which enables a high catalytic activity together with an optimum selectivity to be maintained, furthermore allowing the consumption of the noble metals present in the second catalytic layer to be slowed. The profile of concentration of tin between the two catalytic layers is characterized by a monotonic decrease in concentration of the element within the second layer in the opposite direction to the first layer.

In another embodiment, the first catalytic layer has a specific load of noble metal in the range between 3 and 8 g/m² and the second catalytic layer has a specific load of noble metal in the range between 0.8 and 4 g/m². The inventors have found that loads thus reduced of noble metal are more than sufficient to impart an optimum catalytic activity.

According to another aspect, the present invention relates to a method for obtaining an electrode for evolution of gaseous products in electrolytic cells, for example for evolution of chlorine in alkaline brine electrolysis cells, comprising the following stages:

application to a valve metal substrate of a platinum-free first solution comprising a mixture of iridium, ruthenium and tin, subsequent drying at 50-60° C. and decomposition of said first solution by heat treatment at 400-650° C. for a period of 5 to 30 minutes; repetition of the stage a) until said first catalytic composition is obtained with a desired specific load of noble metal;

application of a tin-free second catalytic solution containing platinum being subsequently dried at 50-60° C. and decomposition of said first solution by heat treatment at 400-650° C. for a period of 5 to 30 minutes;

repetition of the stage c) until said first catalytic composition is obtained with a desired specific load of noble metal.

In one embodiment, the temperature of said thermal decomposition in steps a) and c) is between 480 and 550° C.

In one embodiment, said first solution furthermore comprises titanium.

In another embodiment, said second solution comprises palladium and rhodium on their own or in combination with each other.

In a preferred embodiment of the present invention, the two-layers electrode is subjected to a final thermal treatment. In one embodiment, the final thermal treatment is effected at a temperature between 400 and 650° C., preferably at a temperature of around 500° C., for at least 60 minutes, preferably between 60 and 180 minutes, more preferably between 80 and 120 minutes.

Preferably, the first solution comprises the iridium, ruthenium and tin compounds and optionally the titanium compounds in the form of organometallic complexes. In one embodiment, the organometallic complexes are aceto-hydroxychloride complexes of tin, ruthenium, iridium and optionally titanium, respectively.

Without wishing to be limited to a particular scientific theory, it is possible for the stages a and c for thermal treatment or decomposition of the method described above, together with the elements present and with the concentrations thereof within said first and said second solution, because their coefficient of diffusion is also dependent on the temperature, to contribute to the inter-diffusion of the tin and of the platinum present respectively from the first catalytic layer to the second catalytic layer and vice versa.

According to another aspect, the invention relates to a cell for the electrolysis of solutions of alkaline chlorides comprising an anode compartment and a cathode compartment in which the anode compartment is equipped with the electrode in one of the forms such as described above, used as an anode for evolution of chlorine.

According to another aspect, the invention relates to an industrial electrolyser for the production of chlorine and alkali from alkali chloride solutions, when also lacking biasing protection devices and comprising a modular arrangement of electrolytic cells with the anode and cathode compartments separated by ion-exchange membranes or by diaphragms, where the anode compartment comprises an electrode in one of the forms such as described above used as an anode.

The following examples are included in order to demonstrate particular embodiments of the invention, whose practicability has been amply verified within the range of values claimed. It will be evident to those skilled in the art that the compositions and the techniques described in the examples that follow represent compositions and techniques for which the inventors have encountered a good operation of the invention in practice; however, those skilled in the art will furthermore appreciate in the light of the present description, various modifications could be made to the various embodiments described still giving rise to identical or similar results without straying from the scope of the invention.

EXAMPLE 1

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride and iridium complex acetato-hydroxichloride and having a molar composition equal to 25% Ru, 11% Ir and 64% Sn referred to the metals.

A second solution was also prepared containing a quantity of Pt diamino dinitrate, Pt(NH3)2(NO3)2 corresponding to 40 g of Pt dissolved in 160 ml of glacial acetic acid and then made up to a volume of one litre with acetic acid at 10% by weight.

The first acetic solution was applied to the mesh of titanium by painting on in 8 coats. After each coat, a drying step at 50-60° C. was carried out for around 10 minutes, then a thermal treatment for 10 minutes at 500° C., the mesh being each time cooled in air prior to the application of the next coat.

The procedure was repeated until a load expressed as the sum of Ir and Ru referred to the metals equal to 7 g/m² was reached.

Subsequently, the second solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air before the application of the next coat.

The procedure was repeated until a total load of Pt equal to 2.5 g/m² was reached.

A final thermal treatment at 500° C. for 100 minutes was lastly carried out.

The electrode thus obtained was identified as specimen #1.

EXAMPLE 2

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride and iridium complex acetato-hydroxichloride and having a molar composition equal to 26% Ru, 10% Ir and 64% Sn referred to the metals.

100 ml of a second acetic solution were also prepared containing an organo-metallic complex of platinum and an organo-metallic complex of palladium and having a molar composition equal to 87% Pt and 13% Pd referred to the metals.

The first acetic solution was applied to the mesh of titanium by painting on in 8 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a load expressed as the sum of Ir and Ru referred to the metals equal to 6.7 g/m² was reached.

Subsequently, the second acetic solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Pt and Pd referred to the metals equal to 2.7 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out.

The electrode thus obtained was identified as specimen #2.

EXAMPLE 3

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride and iridium complex acetato-hydroxichloride and having a molar composition equal to 26% Ru, 10% Ir and 64% Sn referred to the metals.

100 ml of a second acetic solution were then prepared containing an organo-metallic complex of platinum, an organo-metallic complex of palladium and RhCl3 and having a molar composition equal to 86% Pt, 10% Pd and 4% Rh referred to the metals.

The first acetic solution was applied to the mesh of titanium by painting on in 8 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a load expressed as the sum of Ir and Ru referred to the metals equal to 6.7 g/m² was reached.

Subsequently, the second acetic solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Pt, Pd and Rh referred to the metals equal to 2.8 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out.

The electrode thus obtained was identified as specimen #3.

EXAMPLE 4

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride, iridium complex acetato-hydroxichloride and titanium complex acetato-hydroxichloride and having a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Ti referred to the metals.

100 ml of a second acetic solution were also prepared containing an organo-metallic complex of platinum and an organo-metallic complex of palladium and having a molar composition equal to 87% Pt and 13% Pd referred to the metals.

The first acetic solution was applied to the mesh of titanium by painting on in 8 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a load was reached expressed as the sum of Ir and Ru referred to the metals equal to 6.7 g/m².

Subsequently, the second acetic solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Pt and Pd referred to the metals equal to 2.7 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out. The electrode thus obtained was identified as specimen #4.

EXAMPLE 5

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride, iridium complex acetato-hydroxichloride and titanium complex acetato-hydroxichloride and having a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Ti referred to the metals.

100 ml of a second acetic solution were also prepared containing an organo-metallic complex of platinum, an organo-metallic complex of palladium and RhCl3 and having a molar composition equal to 86% Pt, 10% Pd and 4% Rh referred to the metals.

The first acetic solution was applied to the mesh of titanium by painting on in 8 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a load expressed as the sum of Ir and Ru referred to the metals equal to 6.7 g/m² was reached.

Subsequently, the second solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Pt, Pd and Rh referred to the metals equal to 2.7 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out. The electrode thus obtained was identified as specimen #5.

COUNTER-EXAMPLE 1

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a hydro-alcoholic solution were then prepared containing RuCl3*3H2O, H2IrCl6*6H2O, TiCl3 in a solution of isopropanol, having a molar composition equal to 23% Ru, 22% Ir, 55% Ti.

The solution was applied to the mesh of titanium by painting on in 14 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the work piece was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Ir and Ru referred to the metals equal to 11 g/m² was reached. Then, a final thermal treatment at 500° C. for 100 minutes was carried out.

The electrode thus obtained was identified as specimen #1C.

COUNTER-EXAMPLE 2

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution at 20% of HCl, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride and iridium complex acetato-hydroxichloride and having a molar composition equal to 26% Ru, 10% Ir and 64% Sn referred to the metals.

100 ml of a second acetic solution were also prepared containing an organo-metallic complex of platinum and a tin complex acetato-hydroxichloride and having a molar composition equal to 87% Pt and 13% Sn referred to the metals.

The first acetic solution was applied to the mesh of titanium by painting on in 6 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Ir and Ru referred to the metals equal to 6 g/m² was reached.

Subsequently, the second acetic solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as Pt referred to the metal equal to 2.5 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out.

The electrode thus obtained was identified as specimen #2C.

COUNTER-EXAMPLE 3

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes.

100 ml of a first acetic solution were then prepared containing tin complex acetato-hydroxichloride, ruthenium complex acetato-hydroxichloride, iridium complex acetato-hydroxichloride and organo-metallic complex of platinum and having a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Pt referred to the metals.

The acetic solution was applied to the mesh of titanium by painting on in 10 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Ir, Ru and Pt referred to the metals equal to 8 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out.

The electrode thus obtained was identified as specimen #3C.

COUNTER-EXAMPLE 4

A mesh of titanium of dimensions 10 cm×10 cm was washed three times in de-ionized water at 60° C., changing the liquid each time. The washing was followed by a thermal treatment for 2 hours at 350° C. The mesh was then subjected to a treatment in a solution of HCl at 20%, boiling for 30 minutes. 100 ml of a first hydro-alcoholic solution were then prepared containing RuCl3*3H2O, H2IrCl6*6H2O, TiOCl2 in a mixture of water and 1-butanol acidified with HCl, having a molar composition equal to 26% Ru, 23% Ir, 51% Ti referred to the metals.

100 ml of a second hydro-alcoholic solution were also prepared containing H2PtCl6 and PdCl2.

The first acetic solution was applied to the mesh of titanium by painting on in 8 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum of Ir and Ru referred to the metals equal to 6 g/m² was reached.

Subsequently, the second acetic solution was applied by painting on in 4 coats. After each coat, a drying step at 50-60° C. for around 10 minutes was carried out, then a thermal treatment for 10 minutes at 500° C. Each time, the mesh was cooled in air prior to the application of the next coat.

The procedure was repeated until a total load of noble metal expressed as the sum Pt+Pd referred to the metals equal to 3 g/m² was reached.

Lastly, a final thermal treatment at 500° C. for 100 minutes was carried out.

The electrode thus obtained was identified as specimen #4C.

The specimens of the examples and of the counter-examples were characterized as anodes for evolution of chlorine in a laboratory cell filled with a brine solution of sodium chloride at a concentration of 200 g/l.

Table 1 reports the over-voltage of chlorine measured at a current density of 4 kA/m² and the percentage by volume of oxygen in the chlorine produced.

TABLE 1 Specimens Cell potential (V) O2/Cl2 (Vol %) 1 2.76 0.9 2 2.76 0.7 3 2.76 0.7 4 2.77 0.8 5 2.77 0.7 1C 2.78 1.2 2C 2.76 1.0 3C 2.77 1.5 4C 2.76 0.8

The specimens of the preceding examples also underwent a test for operation in beaker. In Table 2, the anode potentials (CISEP) are reported, measured in a sodium chloride solution at a concentration of 200 g/I at a temperature of 80° C., corrected for the ohmic drop at a current density of 3 kA/m². Furthermore, in order to evaluate the selectivity for the chlorine reaction, tests were conducted in sulphuric acid at a current density of 3 kA/m²; the anode potentials reported (CISEP) have been corrected for the ohmic drop. The higher the value of the anode potentials measured in sulphuric acid, the greater the selectivity for the chlorine reaction.

TABLE 2 ClSEP in NaCl ClSEP in H2SO4 Specimens vs NHE vs NHE 1 1.336 1.820 2 1.336 1.872 3 1.336 1.890 4 1.338 1.872 5 1.338 1.890 1C 1.347 1.693 2C 1.336 1.740 3C 1.336 1.647 4C 1.336 1.872

Some specimens were, in the end, subjected to a longevity test. The longevity test in question is the simulation, in a cell divided by the conditions of industrial electrolysis. Table 3 reports the cell voltage for the specimens at the start of the test and after a simulated period of a year, as an indicator of their catalytic activity for the evolution of chlorine (Cl O.V.) measured at a current density of 8 kA/m² and the percentage of residual load of the second catalytic layer after a simulated period of a year.

TABLE 3 Cl O.V Cl O.V. after 1 Specimens Start of test year % residual load 2 0.035 0.035 80% 1C 0.050 0.050 — 4C 0.037 0.060 50%

The preceding description is not intended to limit the invention, which may be used according to various embodiments without however deviating from the objectives and whose scope is uniquely defined by the appended claims.

In the description and in the claims of the present application, the terms “comprising”, “including” and “containing” are not intended to exclude the presence of other additional elements, components or process steps.

The discussion of documents, items, materials, devices, articles and the like is included in this description solely with the aim of providing a context for the present invention. It is not suggested or represented that any or all of these topics formed part of the prior art or formed a common general knowledge in the field relevant to the present invention before the priority date for each claim of this application. 

1. An electrode for gas evolution in electrolytic processes comprising a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate containing a mixture of iridium, ruthenium, tin and platinum or their oxides or combinations thereof, obtained from precursors containing said iridium, ruthenium and tin in the form of organometallic complexes, and a second catalytic layer formed on said first catalytic layer containing platinum and tin or their oxides or combinations thereof, wherein said tin of the said second catalytic layer is present in a decreasing concentration from the interface with said first catalytic layer and wherein said platinum of the said first catalytic layer is present in a decreasing concentration from the interface with said second catalytic layer.
 2. An electrode for gas evolution in electrolytic processes comprising a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate containing a mixture of iridium, ruthenium, tin and platinum or their oxides or combinations thereof and a second catalytic layer formed on said first catalytic layer containing platinum and tin or their oxides or combinations thereof, wherein said first layer is obtained from a platinum-free first precursor solution comprising a mixture of iridium, ruthenium and tin, applied said substrate and subjected to a heat treatment, wherein said platinum-free first precursor solution contains said iridium, ruthenium and tin in the form of organometallic complexes, and wherein said second catalytic layer is obtained from a tin-free second catalytic composition containing platinum, applied said substrate and subjected to a heat treatment.
 3. The electrode according to claim 1, wherein said second catalytic layer contains Pt=48-96% in the form of metal, or its oxides, in molar percentage referred to the metal element.
 4. The electrode according to claim 1, wherein said second catalytic layer contains Pd=0-24% or Rh=0-24%, in the form of metal, or their oxides, or combinations thereof, in the form of metals or their oxides in molar percentage referred to the metal elements.
 5. The electrode according to claim 1, wherein said second catalytic layer contains Sn=4-12% in the form of metal or its oxides, in average molar percentage referred to the metal element.
 6. The electrode according to claim 1, wherein said iridium, ruthenium and tin oxides of said first catalytic layer are present in molar percentages Ru=24-34%, Ir=3-13%, Sn=30-70% referring to the metal elements.
 7. The electrode according to claim 1, wherein said first catalytic layer also contains titanium oxides in molar percentage Ti=30-40% referred to the metal element.
 8. The electrode according to claim 1, wherein said first catalytic layer contains Pt=3-10% in the form of metal or its oxides, in average molar percentage referred to the metal element.
 9. The electrode according to claim 1, wherein the valve metal substrate is selected from the group consisting of titanium, tantalum, zirconium, niobium, tungsten, aluminium, silicon, or their alloys.
 10. A method for the production of an electrode as defined in claim 1, comprising the following steps: applying to a valve metal substrate a platinum-free first solution comprising a mixture of iridium, ruthenium and tin, subsequently drying at 50-60° C. and carrying out decomposition of said first solution by heat treatment at 400-650° C. for a time of 5 to 30 minutes, wherein said first solution contains said iridium, ruthenium and tin in the form of organometallic complexes; repeating the previous step until a desired specific load of noble metal is reached; applying a tin-free second catalytic solution containing platinum and subsequently drying at 50-60° C. and carrying out decomposition of said second solution by heat treatment at 400-650° C. for a time of 5 to 30 minutes; repeating the previous step until a desired specific load of noble metal is reached.
 11. The method according to claim 10, wherein the temperature of said thermal decomposition in steps a) and c) is between 480 and 550° C.
 12. (canceled)
 13. A cell for the electrolysis of solutions of alkaline chlorides comprising an anodic compartment and a cathodic compartment wherein the anodic compartment is equipped with the electrode according to claim
 1. 14. A cell for electrolysis according to claim 13 wherein said anodic compartment and said cathodic compartment are separated by a diaphragm or an ion-exchange membrane.
 15. An electrolyzer for the production of chlorine and alkali from alkali chloride solutions comprising a modular arrangement of cells, wherein each cell is the cell according to claim
 13. 16. The electrode according to claim 2, wherein said second catalytic layer contains Pt=48-96% in the form of metal, or its oxides, in molar percentage referred to the metal element.
 17. The electrode according to claim 2, wherein said second catalytic layer contains Pd=0-24% or Rh=0-24%, in the form of metal, or their oxides, or combinations thereof, in the form of metals or their oxides in molar percentage referred to the metal elements.
 18. The electrode according to claim 2, wherein said second catalytic layer contains Sn=4-12% in the form of metal or its oxides, in average molar percentage referred to the metal element.
 19. The electrode according to claim 2, wherein said iridium, ruthenium and tin oxides of said first catalytic layer are present in molar percentages Ru=24-34%, Ir=3-13%, Sn=30-70% referring to the metal elements.
 20. The electrode according to claim 2, wherein said first catalytic layer also contains titanium oxides in molar percentage Ti=30-40% referred to the metal element.
 21. The electrode according to claim 2, wherein said first catalytic layer contains Pt=3-10% in the form of metal or its oxides, in average molar percentage referred to the metal element. 